In our commonly owned copending application Ser. No. 190,167, filed Sept. 23, 1980, now abandoned we have disclosed a plug of this type, molded integrally from synthetic resin, in combination with a recorder whose body including the mouthpiece consists of the same kind of material. The plug, as also known from commonly owned German utility model No. 76 05 033 published July 8, 1976, is a unitary member with a concave front end wall, a flat rear end wall and a plurality of connecting walls therebetween, specifically an upper wall designed to form the flue boundary, a shorter lower wall remote from the flue and a pair of intersecting reinforcing webs respectively parallel and perpendicular to the upper and lower walls. The outline of the front wall, when projected upon the plane of the rear wall, registers with the circular outline of the latter whereby the two end walls define sections of an imaginary cylinder encompassing the outer surface of the lower wall. This wall structure forms four laterally open voids which reduce the weight of the plug as well as the amount of plastic material required for its manufacture; two of these voids, pursuant to the teaching of our copending application, also facilitate the attachment thereto of an insert with a generally U-shaped profile which fits into the flue and comes to rest against an inner peripheral surface of the mouthpiece while leaving an air channel between itself and the plug.
Since synthetic resins are relatively dimensionally stable with changing ambient temperature and humidity, the interfitting of molded plugs and mouthpieces presents no particular problem. When, however, such a plug is to be used with a wooden recorder, the instrument body tends to shrink and swell under varying climatic conditions; its mouthpiece, in particular, also expands inward in response to moist air blown by the player.
Thus, the bore of the mouthpiece designed to accommodate the plug cannot be dependably dimensioned to form a closely fitting seat around the plug. If its inner diameter increases, the plug may not stay securely in place; if it decreases, the presence of the plug may crack the head or fipple of the instrument.